During his recuperation process, Slater began experiencing and reading about paranormal phenomena, including divination (tarot cards), clairvoyance, and levitation.[1][2] He spent an entire year lying in bed in a body cast that weighed 300 pounds. Then one morning, he awoke to find himself stretched across a chair on the opposite side of the room while still in his body cast. These experiences led him to witchcraft, and in 1972, he was initiated into the New York Coven of Welsh Traditional Witches.[1] It was there he met Eddie Buczynski (Lord Gywddion), who was a high priest of the coven.[1][2] Slater took the craft name Lord Govannon and the two became lovers.[2]

Bucznski and Slater opened the The Warlock Shoppe, the oldest witchcraft bookshop in Brooklyn, New York.[1] Buczynski was the more magical and spiritual of the two and left the business side to Slater, who helped the shop grow in profit. Most importantly, the shop established itself as the central information hub for local witches and the newly emerging neopagan communities.[2] The two also published a periodical called Earth Religion News.[1] It was extremely successful but also caused controversy due to its explicit contents and cover designs.[2] In 1974, Slater was initiated into the Gardnerian tradition and assumed leadership of the coven in the late 1970s.[1] The Warlock Shoppe later moved to West 19th Street in Manhattan (the borough of New York City) and operated under the name Magickal Childe. The Magickal Childe functioned as a major focal point for the neopagan community in the 1970s and well into the 1990s. In the later 1980s it gained something of a mercenary reputation being willing to put 'curses' on people for a price. with Slater's death they started having trouble making ends meet and several significant new age publishers stopped providing them with books. The brick and mortar store finally closed in 1999.

In 1972, Slater presented the Inquisitional Bigot of the Year award to NBC during a guest appearance on the Today show, for an episode of Macmillan and Wife that had taken witchcraft and corrupted it into devil-worship rituals for the plot.[1] The crew of Today had Slater physically removed from the set.[1] More controversy surrounding Slater's actual proficiency in the types of magick he claimed to practice, accusations that he plagiarized material, yelling out at irritable customers in his Magickal Childe store, "Get out of my store...", as well as outrage over other behaviors he exhibited earned him the nickname "Horrible Herman".[2]

These two witchcraft cookbooks are based on the inner workings of his shop and formulas of his potions. They are sold worldwide and are well-respected within the witchcraft community.[9] The Magickal Childe now continues with an Internet presence.[10]

Educating people on the subject of witchcraft became an important mission for Slater. He frequently lectured as a guest speaker at many colleges. He starred in his own video, An Introduction to Witchcraft and Satanism, in which he wore ceremonial robes and headdresses typical of witchcraft. He also appeared with his familiar companion, a snake named Herman. He also hosted a weekly cable show which aired in Manhattan, called The Magickal Mystery Tour. The show featured interviews, rituals, music, occultism, and magick instruction. He thought of the show as an Earth religion 700 Club because it spread the word on the Old Religion and asked for donations.[1]

Slater was a self-professed conservative who opposed drugs, promiscuity, and love magick. He criticized witches and pagans who indulged in drugs and sex magick. His blunt opinions made him unpopular.[1] Despite his outward appearances and persona, he did not practice what he preached. Slater was known for keeping many lovers outside of his relationship with Buczynski.[2]

One of Slater's former employees described him as "Very bright, almost schizophrenic, fiercely loyal one moment, then your brutal enemy the next. I loved and hated him so many times in turn that it's all a blur."[11] However, many loyal followers were befriended by Slater, including most of his loyal employees, loving family and friends.[12]